Rename PXEGen class to TFTPGen

PXE refers to DHCP + TFTP. PXEGen name seems that this class would
generate DHCP conf file as well and it does not; this is confusing.
Rename class to make it clear.
Merge pull request #1214 from alanoe/alanoe/rename_pxegen

Rename PXEGen class to TFTPGen